1. Field of the Invention
The present invention relates to a position detecting device that is able to prevent from detecting falsely a position and avert damage by external vibrations.
2. Description of the Related Art
For example, JP(PCT)7-500421A discloses a position detecting device including a magnetic field generator which generates a magnetic field; a holding member which connects the magnetic field generator and an object whose position is to be detected (to-be-detected object), and is movable together with the generator in the position detecting direction; a first magnetic substance and a second magnetic substance which are arranged opposite to each other with a predetermined gap left therebetween so as to sandwich the holding member; a supporting member which supports these first magnetic substance and second magnetic substance; and a magnetic sensor which is inserted in a gap formed in either of the first magnetic substance and the second magnetic substance, and detects a position of the holding member, i.e., the to-be-detected object based on a magnetic flux passing through the magnetic substance into the gap of which the sensor is inserted, for detecting a position of the to-be-detected object that is to be in linear motion.
The magnetic field generator is magnetized so as to generate a magnetic field forming a clockwise magnetic path and a counterclockwise one with respect to each of the first magnetic substance and the second magnetic substance, and the positions of the magnetic paths shift depending on where the magnetic field generator is poisoned (i.e., a position of the to-be-detected object). The shift of the magnetic path changes a density and a direction of the magnetic flux passing through the magnetic sensor, and the magnetic sensor outputs a signal corresponding to the density and the direction of the magnetic flux. The output signal is a signal indicative of the position of the object to be detected and has a linear relationship to the position. Therefore, an output signal of the magnetic sensor enables to detect where an object is moved.
Moreover, the magnetic field generator is secured to the holding member so as to be disposed in the center of the gap between the first magnetic substance and the second magnetic substance.
Further, the holding member to which the magnetic field generator is secured slidably moves through a sliding part provided on the holding member along sliding parts provided on the supporting member supporting the first magnetic substance and the second magnetic substance. In this case, each side of the first magnetic substance and the second magnetic substance participate in the slide.
Since the conventional position detecting device is arranged as mentioned above, and particularly the magnetic field generator is disposed in the center of a gap between the first magnetic substance and the second magnetic substance, strengths of the magnetic force generated between the magnetic field generator and the first magnetic substance, and between the generator and the second magnetic substance are almost equal. For this reason, the magnetic field generator will be placed in an unstable position to an external force. The generator is susceptible to a swing, e.g., when a vibration is applied from the outside. Specifically, when a vibration is applied in the position detecting direction, the generator swings and the holding member cannot keep connection with the object to be position-detected in the worst case, resulting in false detection of the position.
Moreover, when the generator swung, e.g., in the direction of the first magnetic substance or the second magnetic substance, the generator collides against the first magnetic substance or the second magnetic substance in the worst case, or else the holding member which holds and secures the generator collides with the supporting member which supports the first magnetic substance and the second magnetic substance. This can damage the generator according to circumstances.
As a method of preventing the generator from being swung, there can be a method of applying a large load to the generator in advance in the position detecting direction in the event that an external force is applied, and a method of reducing spaces between the generator and the first and second magnetic substances, and between the holding member and the first and second magnetic substances as much as possible such that the generator or the holding member does not greatly deviate from the original position detecting direction. However, when these methods are taken, it brings about an unduly raised production cost and lowered productivity because these methods require an addition of load applying means and an increase in the dimensional precision of the components.